1. Field of the Invention
This invention relates to advancing and working thermoplastic materials to produce homogeneous extrudate, and, more particularly, to apparatus for advancing expandable thermoplastic materials successively through feed, compression, relief and metering zones of an extruder for producing cellular plastic insulating and jacketing materials with facilities being provided in the compression zone, which do not interrupt th helical flight of the extruder screw, for maintaining a substantially uniform thermal history for successive sections of melt at an output end of the metering zone.
2. Description of the Prior Art
In the extrusion of thermoplastic materials for insulating conductors for communications systems, there is a desire for higher line speeds, thereby necessitating higher extruder output. The output rates for extruders are limited by the requirement of obtaining a uniform extrudate.
The thermoplastic material begins to melt along the interface with the inner surface of the extruder barrel. Once melting has begun, three distinct regions are noted in a cross section of a channel formed by a helical flight of an extruder screw. These are (1) the unmelted plastic or solid bed, (2) a thin melt film between the solid bed and the barrel, and (3) a melt pool where melted material collects.
The term "solid bed" refers to the plastic material prior to a transformation into a substantially less viscous melt material. The solid bed generally remains intact up to a point within a compression section of the screw where it will rupture. The later the solid-bed break-up within the compression section, the more desirable is the screw design. As portions of the solid bed are broken off from the initial mass, the portions flow downstream of the screw and continue to melt. Then as the helical flight of the extruder screw advances, the flight wipes off the melt and forms a melt pool on the downstream side of each section of a channel formed by the turns of the flight.
At some location in the compression section, the solid bed breaks up into large portions. The location of the solid bed break-up and size of the portions are a function of screw design and operating conditions. As the solid bed heats up, the plastic is transformed into a very viscous melt surrounded by less viscous melt. The more viscous melt may resist mixing and transformation into a less viscous form thereby detracting from the homogeneity and thermal exposure of the mix.
Improved mixing and temperature distribution have been achieved by using extruders having increased barrel length-to-diameter ratios. The evolution of extruder screw designs is discussed in U.S. Pat. No. 3,762,693 issued in the names of R. V. DeBoo and C. B. Heard Jr., incorporated by reference hereinto. Terms such as "mixing", "dispersing" and "flight diameters" are terms well known in the art and are defined, for example, in U.S. Pat. Nos. 3,530,534 and 3,762,693.
Slotted ring design screws with pins extending into the channel in a metering section to cause previously broken solid bed to further break up and thereby increase conduction heat melting are exemplified by U.S. Pat. No. 3,486,193. This design is characterized by a broken flight to permit mounting the pins continuously around the root diameter section of the screw within the metering section. This causes undesirable "dead spaces" which tend to cause a backup of the thermoplastic material.
In U.S. Pat. No. 3,487,503, a multiplicity of pins are arranged crosswise or lengthwise of the channel in any region such as the metering or compression sections in which the material is received in a molten or plastic condition to achieve efficient mixing of the thermoplastic material within the extruder resulting in greater uniformity in the extrudate. Although some of the pins in any one turn of the flight of the screw lie in a plane which may be perpendicular to the axis of the screw, other ones of the pins in that turn of the flight lie outside the plane. Pins arranged in this manner have been found to introduce excessive restrictions to flow thereby necessitating increases in the RPM of the extruder screw and shear heat which are critical for control of temperature and hence expansion of a blowing agent when working expandable thermoplastic materials.
A uniform solid insulation extrudate has been achieved by using a pin arrangement as disclosed in U.S. Pat. No. 3,762,693, referred to hereinbefore, where the metering section of the screw is provided with at least one group of pins, all of the pins in any one group lying in a plane perpendicular to the axis of rotation of the screw. These pins facilitate further breakup of solid bed which reaches the metering section and provide small portions with increased surface area to increase the effect of conductive heat upon the plastic material.
While the above arrangements, and particularly the last described, have been found suitable for working the usual thermoplastic materials and achieving a uniform extrudate at the die end, problems arise when extruding cellular insulation. There, unlike normal solid insulation, it is desired that the thermal history, and not just the temperatures of corresponding portions of successive sections of the extrudate, be controlled to insure uniformity of expansion. This will minimize fluctuations in the coaxial capacitance of the insulated conductor (hereinafter referred to as "capacitance") and diameter-over-dielectric (hereinafter referred to as "DOD").
Conventional extruder screws provide undesirably a premature as well as an intermittent rather than a continuous fragmenting of the solid bed of thermoplastic material. This presents a problem for thermoplastic materials having a chemical blowing agent priorly introduced thereinto. The blowing agent in the broken solid bed is not exposed to as high a temperature for as long a period of time as the blowing agent in the melt between consecutive pieces of solid bed. A low temperature history of the blowing agent in the solid bed will cause decreased percent expansion. Variations in the percent expansion causes variation, undesirably, in the capacitance and DOD of the conductor insulation.
Problems encountered in extruding cellular insulation have been recognized. In U.S. Pat. No. 3,287,477, spaced portions of the screw are provided with longitudinal grooves to form "choke" sections. There is still a need for facilities to control both the location of the solid-bed breakup of cellular insulation material and the manner in which it is broken up.